Description: When the client/server reaches the BOUND state, this is one of the parameters set by the server.

Mentioned in RFC2131 4.4.5 as T2,this Expiration time period in secs is the time period in secs during which DHCP has gone through the BOUND->RENEWAL state. After T1 secs and when the state machine reaches T2 secs, ( T1 < T2 < lease period), DHCP client sends another DHCPREQUEST to the server asking the server to renew the lease for the IP parameters. By default it would be 87.5% of the Lease timer .If there is DHCPACK then the DHCP client moves from REBIND to BOUND. Else it moves to INIT state where it starts all over again sending a request for DHCPDISCOVER.